Using Mockito to mock classes with generic parameters

I agree that one shouldn't suppress warnings in classes or methods as one could overlook other, accidentally suppressed warnings. But IMHO it's absolutely reasonable to suppress a warning that affects only a single line of code.

@SuppressWarnings("unchecked")
Foo<Bar> mockFoo = mock(Foo.class);

As the other answers mentioned, there's not a great way to use the mock() & spy() methods directly without unsafe generics access and/or suppressing generics warnings.

There is currently an open issue in the Mockito project (#1531) to add support for using the mock() & spy() methods without generics warnings. The issue was opened in November 2018, but there aren't any indications that it will be prioritized. Per one of the Mockito contributor's comments on the issue:

Given that .class does not play well with generics, I don't think there is any solution we can do in Mockito. You can already do @Mock (the JUnit5 extension also allows method parameter @Mocks) and that should be a suitable alternative. Therefore, we can keep this issue open, but it is unlikely ever to be fixed, given that @Mock is a better API.

With JUnit5 I think the best way is to @ExtendWith(MockitoExtension.class) with @Mock in the method parameter or the field.

The following example demonstrates that with Hamcrest matchers.

package com.vogella.junit5;                                                                    
                                                                                               
import static org.hamcrest.MatcherAssert.assertThat;                                           
import static org.hamcrest.Matchers.hasItem;                                                   
import static org.mockito.Mockito.verify;                                                      
                                                                                               
import java.util.Arrays;                                                                       
import java.util.List;                                                                         
                                                                                               
import org.junit.jupiter.api.Test;                                                             
import org.junit.jupiter.api.extension.ExtendWith;                                             
import org.mockito.ArgumentCaptor;                                                             
import org.mockito.Captor;                                                                     
import org.mockito.Mock;                                                                       
import org.mockito.junit.jupiter.MockitoExtension;                                             
                                                                                               
@ExtendWith(MockitoExtension.class)                                                            
public class MockitoArgumentCaptureTest {                                                      
                                                                                               
                                                                                               
    @Captor                                                                                    
    private ArgumentCaptor<List<String>> captor;                                               
                                                                                               
    @Test                                                                                      
    public final void shouldContainCertainListItem(@Mock List<String> mockedList) {            
        var asList = Arrays.asList("someElement_test", "someElement");                         
        mockedList.addAll(asList);                                                             
                                                                                               
        verify(mockedList).addAll(captor.capture());                                           
        List<String> capturedArgument = captor.getValue();                                     
        assertThat(capturedArgument, hasItem("someElement"));                                  
    }                                                                                          
}                                                                                              
                                                                                              

See https://www.vogella.com/tutorials/Mockito/article.html for the required Maven/Gradle dependencies.

JUnit5: use @ExtendWith(MockitoExtension.class) on the test class then add this field:

@Mock
Foo<Bar> barMock;

So you have this:

Foo mockFoo = mock(Foo.class);

Ways to fix it, starting from my least favourite to most:

1. Use @SuppressWarnings("unchecked") annotation. Doesn't really fix it, but you'll stop getting the warnings.

@SuppressWarnings("unchecked")
Foo mockFoo = mock(Foo.class);
when(mockFoo.getValue).thenReturn(new Bar());

2. Cast it. Though it still gives warnings, unfortunately. So you need to use the annotation here as well:

@SuppressWarnings("unchecked")
Foo<Bar> mockFoo = (Foo<Bar>) mock(Foo.class);
when(mockFoo.getValue).thenReturn(new Bar());

3. Use @Mock annotation. There will be no warnings. Here, when can be added in actual tests.

@Mock
public Foo<Bar> fooMock;

4. Use @MockBean annotation. This will create a mocked bean directly. No warnings.

@MockBean
public Foo<Bar> fooMock;

why not using spy

var mock = spy(new Foo<Bar>());
when(mockFoo.getValue()).thenReturn(new Bar());

The (in my opinion) most easiest and most readable approach is to use method level injection.

This will result in having all test data within the test method. This will keep your test classes clean as there are no 'floating' mock's.

@ExtendWith(MockitoExtension.class)
public class SomeClassTest {

    @Test
    void someTestMethod(@Mock Foo<Bar> fooMock) {
        // do something with your mock
    }
    
}